Fixing injection of Readium CSS's default stylesheet

[mickael-menu]

Readium CSS's default stylesheet is currently systematically injected, but Readium CSS states that it should only be injected when the publication doesn't have styles.

The following modules must be appended if there is no external stylesheet (<link>), internal stylesheet (<style>) or inline styles in the document (style=" "), in this exact order:

1. ReadiumCSS-default.css

This causes faulty rendering in the EPUB navigator: